The threat of inhaled Bacillus anthracis (B. anthracis) as an agent of bioterrorism is an important problem in the United States today. Systemic B. anthracis following pulmonary inhalation results in shock and pulmonary edema and is fatal in almost 50% of cases despite effective antibiotic and other support. B. anthracis produces 2 toxins; lethal toxin (LeTx) comprised of lethal factor (LF) and protective antigen (PA), and edema toxin (ETx) comprised of edema factor (EF) and PA. Much data supports a central role of LeTx in the pathogenesis of B. anthracis. Lethal factor is a zinc protease that could inactivate members of the mitogen-activated protein kinases (MAPKKs or MEKs) family and subsequently inhibits phosphorylation and activation of downstream mitogen-activated protein kinases (MAP kinases) such as extracellular signal regulated kinase (ERK), c-Jun NH2-terminal kinase/stress-activated protein kinase (JNK/SAPK) and p38. Mutant strains lacking LeTx are much less virulent and the direct administration of LeTx is lethal in many animal models. Work with LeTx has been facilitated by the availability of recombinant forms of the protein. Despite its likely importance in the pathogenesis of B. anthracis however, administration of LeTx in animal models, while producing shock and lethality, does not result in the characteristic hemorrhagic tissue injury found in patients dying with this infection. This has suggested that virulence factors in addition to LeTx, such as ETx, may contribute to the pathogenesis of this microbe. Although B. anthracis mutants lacking ETx have been reported to retain virulence, studies support a role for this protein in the pathogenesis of the disease. ETx is an adenyl cyclase that transforms ATP to cAMP and can alter immune and other responses by the host. Past studies in some animal models with early preparations of this toxin had suggested that ETx would add to the effects of LeTx. However, studies with ETx were hampered by lack of recombinant preparations. Such preparations are now becoming available. A critical question at this time, both for better defining the pathogenesis of B. anthracis as well as its treatment is to understand how ETx functions alone and in combination with LeTx. In the present experiments, recently available preparations of recombinant ETx and LeTx were investigated. We first compared the effects of increasing concentrations of LeTx and ETx alone. In these experiments, ETx in doses less than a log less than LeTx resulted in hypotension and lethality similar to LeTx. Thus, ETx alone appears to have potentially lethal effects at concentration comparable to lethal doses of LeTx. We then tested the effects of ETx and LeTx in combination and found that their effects are additive but not synergistic. In a set of experiments presently ongoing, we are describing the effects of similarly lethal doses of ETx and LeTx either alone or in combination on cardiopulmonary and histology changes and the host inflammatory and defense responses. In a final set of planned experiments, we will investigate the effects of a protective antigen directed monoclonal antibody with ETx and LeTx administered alone or in combination.